DE2846543A1 - PLAYBACK WITH A LIQUID CRYSTAL - Google Patents

Description

9/22/197s y PHN 8 92

Display device with a liquid crystal

The invention relates to a display device which includes a display screen with a number of display elements and an excitation circuit, which display starts _with: a liquid crystal, a first cover plate for the liquid crystal with a number of first electrodes on a side of the first cover plate facing the liquid crystal, a second Cover plate is provided with a number of second "electrodes" on a side of the second cover plate facing the liquid crystal and with optical aids for viewing a Riclitz state of groups of molecules of the liquid crystal, the first and second cover plates being almost parallel to one another and at the edges with sealing means for enclosing the liquid crystal between the cover plates, and wherein a display element is formed by a part of the liquid crystal which is sandwiched between a part of a first electrode opposite! laying part of a second electrode, and which excitation circuit with a first generator for generating a first alternating voltage with a high frequency, a second generator for generating a second alternating voltage with a low Fj equöDE-, a 30.IeICtXOnKiJCjIa-ItUiIg for alternating .Kopp η j π the first genarai .ovs: uiv at least one first 3/1 «; \ ί .roda ■ gjjiüj ^{1 control} scarf iu.ag

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for coupling the first generator to the second electrodes and a coupling circuit for constantly coupling all of them Reproduction elements is provided with the second generator, for a reproduction element during the time in which the first electrode corresponding to this display element is coupled to a first generator, the alternating voltage on the second electrode corresponding to this display element with the alternating voltage on the first Electrode for setting a first directional state of the molecules of the liquid crystal of this display element in phase and for setting a second Richtz ^^ stand is in antiphase.

Display devices of the type mentioned above are used, inter alia, for displaying alphanumeric characters and other characters and / or figures for information processing arrangements used, whereby the reading of the information takes place with the help of optical aids such as polarizers, whereby one of the Riclitz states of the molecules of a display element one dark part of the image and the other directional state one

^ bright part of the picture. In the case of conventional display arrangements, the incident light can be read with any light, v; o to at least one cover plate with the electrodes arranged on it is transparent.

A display device of the type mentioned above is from Alt and Pleshko, IEEE Transactions on Electron Devices, Issue ED-21, No. 2, February 19? 4, pages 1 ^ 6 up to and including 155 known.

In the above article, on page 152 ff Display device of the above type described, the

^ is stranded with a display screen of, for example, the field effect type. In the description of this Wiedei'gabeanordnung on the basis of static sensitivity curves of Viedergabeschinas two effective voltages V and V __ _s are defined in which "the use of two possible RicliT..sust" äijdfi deliver the molecules of liquid crystal required for reading mindρsLens optical contrast. It turns out that the maximum number m of time-division multiplexed groups of playback

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element depends on the relative steepness of the sensitivity curve from which V and V _ are derived, where for a usable contrast m must be limited to a few tens.

The invention now has the task of creating a display device in which a high contrast is possible 'is at very high values of m, even for display screens with a low relative steepness, without this being very requires high control voltages or a complicated excitation circuit.

To solve the problem mentioned, form in a display device of the type mentioned according to the Invention the cover plates with the liquid crystal one Twisted nematic type display screen and the

^ Liquid crystal is of a type that the liquid crystal has a dielectric relaxation, the difference between the dielectric constant in the mean direction of the long axes of the molecules and the dielectric constant in a transverse direction Direction at the high frequency of the first alternating voltage corresponds to the sign of this difference at the low Frequency of the second alternating voltage has opposite signs.

This leads to the surprising result that the maximum value of m no longer differs from the selected values of V and V _ "is dependent. As with the display arrangement from the above article take the required amplitudes of the alternating voltages with the size from m to, which ultimately reduces the breakdown voltage of the

Display screen puts the value within a practical limit, so that it turns out that numbers from in a few hundred to about a thousand with adequately lower Voltage reached v / earth. This enables display arrangements with picture line numbers such as these for

Cathode ray tube display systems are customary in information processing systems, whereby characters with a 7x9 matrix can be displayed in, for example, kO text lines, for which at least 9 x ^ O = 3 ^ 0 controllable image

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lines are required.

In a display device according to the invention, each molecule is exposed to three moments. Firstly, this is a mechanical moment that has arisen from pre-machining of the cover plates, as a result of which the molecules, in the absence of other moments, will align themselves approximately parallel to the cover plates with a drill of 90 °, for example, determined by the pre-machining, namely running from the first to the second cover plate. A second moment is multiplied by the square of the rms value of the alternating voltage with the low frequency by the difference Δ c. = <*. .. _ £. . and by the angle between the direction of the electric field and the mean direction of the long axes of the molecules. For the low frequency, the moment tries to bring the molecules into a rectilinear state, where they run roughly parallel to the direction of the field, further referred to as the "ON" state. The third moment is multiplied by the square of the effective value of the alternating voltage with the high frequency by the difference Λ £ = * L ·. ,, - * £ ■ i “ and determined by the angle between the direction of the electric field and that of the molecules. Since Ai _{1 has} the opposite sign to ^Λ £ _τ , this moment tries to direct the molecules across the direction of the electric field.

The straightening state, with the third moment predominating, becomes further referred to as the "OFF" state.

Because the alternating voltages with the high frequency at the first electrode and the second electrode during the selection time. in which the first electrode is coupled to the first generator in accordance with a display element, fed in phase or in opposite phase can be the effective value of the average on Reproduction element present alternating voltage with the high frequency depending on the information to be reproduced can be set in such a way that said effective value mehl 'or that of the alternating voltage with the low frequency less offsets.

In its simplest form, V is only given to one of the electrode

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sets, for example only every second electrode, the AC voltage of the high frequency superimposed on these electrodes, supplied.

Since only the rms value plays a role, the form of the alternating voltages is unimportant, but often Square wave voltages are used because of the use of digital circuits.

With regard to the life of the display screen it is undesirable for the AC voltages to have a DC component exhibit.

To achieve a good result, there are many combinations of amplitudes of the respective alternating voltages possible. A favorable embodiment, with the tensions are on average as low as possible, are formed by a display device, the display elements are excited by the excitation circuit in time division multiplex, and each of a number m of first electrodes in one of the groups Display elements correspond and the selection circuit couples the first electrodes alternately and in cyclic order with the first generator, characterized in that that the amplitude of the alternating voltage with the high frequency at a first. Electrode by a factor corresponding to the Square root of m higher than the amplitude of the alternating voltage with the high frequency at a second electrode.

An embodiment of the invention is shown in the drawings and is described in more detail below. Show it

Fig. 1 is a timing diagram of the amplitudes and phases of the voltage applied to the electrodes of a display element are supplied,

2 shows a simplified timing diagram of the connection of a matrix-like display device with three rows and three columns,

Fig. 3 © in a simplified block diagram of a

^ ⁰ inventive excitation circuit for controlling a ü: atrix-like display device with m rows and k columns "

In Fig. 1, line 1 is the AC voltage that is applied to the first electrode. Become pex'iodic

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during the period segments t ../ \, tw ₁ \ etc., a number of periods of the alternating voltage with the highest frequency are supplied to the first electrode. Line 2 shows the alternating voltage with the highest frequency as it is fed to the second electrode and line 3 shows the voltage between the two electrodes, with the superimposition of a signal of the lowest frequency being disregarded. Line k shows the alternating voltage of the lowest frequency as it occurs between the electrodes. The alternating voltage can be fed to the first or to the second electrode as desired or also to both electrodes in antiphase, specifically by means of adding circuits for analog signals known per se. The alternating voltages with the highest frequency are shown as square-wave voltages, because these are most easily produced with the aid of logic switching elements, but this is not essential for the idea of the invention. ι

In the example shown, the amplitude of the alternating voltage at the first electrode has been selected to be equal to the value V _c and symmetrical to ground, as have the other voltages. The symmetry is desirable because of the service life of liquid crystals, which service life can in some cases be considerably shortened as a result of DC voltage components in the control.

The amplitude V of the alternating voltage of the highest frequency at the second electrode is in this example smaller than V and about as large as the amplitude V. the alternating voltage of the lowest frequency between the electrodes.

3 "During the first section of the perxode, tw \ one

1 (n)

Period t / \ are the voltages denoted by 1 and 2 in phase opposition and the alternating voltage 3 then has an amplitude equal to V_ * V_. In the remaining part of the period t / \ are the tensions 2 and 3 are equal to each other because the

AC voltage 1 is then equal to zero. That the alternating voltages 2 and 3 in the remaining part of the period t / \ possibly skip one or more times in the phase is of no importance for the display element in question.

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The effective value V _{TT of} the alternating voltage with the highest

xl

Frequency on average over the period t / \ is now greater than V _n . If t ₁ = -t. is then is

J- * -1 I * " 1

v ^{2 m} ~ ' \ r ^z j. (λτ, ν \ ² -L ^V H (n) ⁼ m * ^V D ⁺ ^ S ⁺ V * m *

The same situation occurs in the subsequent period on, where the voltages 1 and 2 during t / \ also in Are antiphase.

In the period shown afterwards, voltages 1 and 2 are in phase, so that voltage 3 now has the amplitude V_-V_ has.

O XJ

So now applies:

v ² - ^m ~ T v ² - ( ν λτ \ ²

With the selected values of V ^ .. ^ and V _4TT ", which the

ILxJM AUb

Correspond to values V and V "_ from the article mentioned, 4g on off

the voltages V, V._ and V ₁ . now chosen such that

m ~ ' D ⁺ m ^ S ~ D ^ and t, "

L ^ AUS ⁼ ^ Hi m * ^V D ⁺ in ^ S ^ D ^ O '""L' L is now assumed, for convenience, that:

- V ² ) + Γν ² - (V -V) ² }

s ο will:

V ² _ EZ-L (γ ²

A ~ m * ^v L

v ² = ² ^ (V ² - V ² ) + - Γ V ² -

OFF ei * ^V LD ^y mLL

If aiun is subtracted, it follows: s kV ' V _x ,

v ² _ ν ² - '-—§ -

ON OFF m. .

and by adding:
30th

V ² '+ V ² - ν ² - V ² - - V ²
ON OFF 'LD m S

Given a value for V _T , this is the LSoung

Lj

zv / ei equations given du.rch:

V ² V ² V ² / V ² V ² V ²

_J5 1 / L -,! THE END ,. τ \ // L ·, / AUSv ₂ m / AUS _n

- 2 ^. -2-2 "i + 2 VV ~~ 7 ~ -2-2 ~ 7 '~ T _t V' ~ ~ ^

γ ^ ν ν ν ν ν

ON OFF ON ON ON ON

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Iv ² ' -; + 2 ™ y ^ γ ~ -2-2

ν · V V

ON ON ON

The lowest value of V for which a solution is possible

JLj

5 is given by:

/ L_ J. JL AUS x ₂ m /

V ² V

ON ON

or V ²

10 -

V ²

A where:

V ² V ²

D 1 I / - Z ₁ OFF

15 _V 2 _V

ON ON

A 20

This choice of V. therefore includes:

vl v ²

5 D

= m

V ² V ²

ON ON

and consequently V = fin. V _T

In the case of liquid crystals of the type used, there is a cut-off frequency, for which i \ Z. = O. This cut-off frequency is usually in the order of magnitude of a few kHz. The low frequency must be selected to be low compared to this cut-off frequency, the high to be high. In the case of a liquid crystal with a cut-off frequency of about 5 kHz, good results are achieved with the low frequency in the order of magnitude of 50 Hz to 1 kHz and with the high frequency in the order of magnitude from 10 kHz to 1 MHz.

In Fig. 2, a display device with 3x3 display elements is shown in a schematic manner. Lines 10, which _{are labeled I 1} , 1 "and 1", represent the group conductors, the first electrodes of the

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Display elements 11, 12, 13; Connect 21, 22, 23 or 31, 32, 33 to one another. The rendering elements are located at the point of intersection of rows 10 with rows 35 'represent the column conductors that form the second electrodes of the Display elements 11, 21, 31: 12, 22, 32 or 13, 23, 33 connect with each other.

The first and second electrodes are supplied with alternating voltages with the highest frequency corresponding to the state described in FIG. 1. For the sake of clarity, the supply of the alternating voltage is with the lowest frequency not specified here.

In a schematic way, a display pattern is designated as an example, the display elements being the "ON" must be indicated by a circle and the rendering elements. which must be "OFF" by a cross. In this example, the display elements 11, 23 and 32 are intended as "ON", the rest as "OFF".

In the left part the voltages with which this is achieved are given. Lines 1-1, 1-2 and 1-3 show the alternating voltages as they _{are fed to the group conductors I 1} , 1 "and 1", lines 2-1,2-2 and 2-3 those which are fed to the column conductors k ", k" and k "are supplied.

During the period t, \, the so-called picture line time of the display device, the voltages 1-1 and 2-1 for the display element 11 are in phase, the effect of V is predominant and 1-1 is "ON". The voltages 2-2 and 2-3 are in antiphase with 1-1 during tw \, so that the display elements 12 and 13 are "OFF". In the period segments t "/ \ and t" / \ of the period time t / \, the so-called picture time in which a complete picture is produced, it can be seen from the figure that the other display elements are also correct. can be controlled.

In these periods, the voltage 1-1 is equal to zero and the voltages 2-1, 2-2 and 2-3 have phases corresponding to the phases that are desired for the display elements to be controlled in a certain period segment are. For elements 11, 12 and 13 the phase is not of

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Meaning, as was already indicated in the description of FIG., And shows for the reproduction element described outside of the period segments t ^ any phase, i.e. any phase with respect to this element.

S In a corresponding manner, the control can take place with the aid of an excitation circuit with m picture lines and k columns, wherein the excitation circuit the m lines 10 individually scanned and k lines 35 provided with voltages, as is desired for a particular ¥ ¥ iedergabemuster.

The integration time over which the effective alternating voltages are averaged depends on the material properties of the liquid crystal. These times when the control of a reproduction element occur are in the order of magnitude of the natural fall time in which excited state of such a crystal. For display devices for the display of, for example, alphanumeric Characters for information processing systems are allowed much longer time constants because of the rendering patterns do not change quickly. Time constants of 100 msec or more can then be used.

3 shows a possible embodiment of an excitation circuit for a matrix-type display device or a display device for digits and other characters made up of segments, each display element forming a segment and the display elements in one

Matrix circuit connected, but mechanically not more essential

are arranged in a rectangular grid pattern.

In this excitation circuit, an output 40 of a line oscillator 42 is coupled to an input 44 of a counter circuit 46 and an input 48 of a multiplexer circuit 50. Dip counting circuit has for a matrix with m picture lines at least m different possible positions, which can be reproduced, for example, at a number of digital outputs 52, many numbers must be at least log m, rounded up to a whole number. The outputs 52 of the counting circuit are connected to corresponding inputs $ h of a 1-out-of-m decoding circuit 56 with m outputs 58-1 up to and including 58-m. Everyone

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Output 58 is connected to a corresponding first input 60 of AND gate circuits 62-1 up to and including 62-m, of which all second inputs 64 are connected to one another and coupled together with an output 66 of a generator 68 for generating an alternating voltage with the highest frequency . The outputs 70-1 up to and including 70-m are connected to only one input 72-1 up to and including 72-ni corresponding excitation amplifiers 74-1 up to and including 74-m, of which the outputs 76-1 up to and including a

JO finally 76-m with the group leaders 10-1 up to and including 10-m of the display device 80 are connected. The operation of this part is as follows. The line oscillator 43 emits counting pulses, specifically at the desired sampling frequency, to the counter h6, which thereby cycles through positions that differ from one another in a cyclical order. These counter positions are coded out by the decoding circuit 56 in a manner known per se in such a way that the outputs 58-1 up to and including 58-m are excited individually one after the other in a cyclical sequence.

As a result, the first inputs 60-1 up to and including 60-m of the AND gate circuits 02-1 up to and including 62-m are excited in the same order. During the excitation time t of an AND gate circuit 62-n (n = 1, 2,... M), the output 70-ri of this AND gate circuit follows the pulse shape of the alternating voltage generated by the generator 68. The pulse sequence produced in this way is amplified to the desired amplitude V by the excitation amplifier Th-ii , namely symmetrically with respect to ground, which amplified voltage is fed to the group conductor 10-n. As soon as and as long as another AND gate circuit 70 is energized, the signal at the group conductor 10-n is equal to zero, corresponding to the illustration of the simplified situation according to FIG. 2.

A second input 82 of the MnitLp1ex circuit 50 is with an output 84 of an information processing Eixüieit 86 connected. The multiplexor circuit is setting up known manner during each picture line time η the binary information that is used for the rendering elements that 'belong to the group leader 10-n', is required to

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Outputs 88-1 up to and including 88-k, which correspond to the column conductors 35-1 up to and including 35-k of the display device 80. Each output 88 of the multiplexer circuit 50 is connected to a corresponding first input of a number of NOT-exclusive-OR gate circuits 92-1 up to and including 92-1c, of which second inputs 9h are connected to one another and coupled together to the output 66 of the generator 68. A NOT-exclusive-OR gate circuit 92 is energized when the inputs 90 and 9 ^ · of this circuit 92 are both energized and when these two inputs are not energized. The output 96 of the circuit 92 thus follows the form of the alternating voltage of the highest frequency of the generator 68 and is thus in phase when the associated input 90 is excited and in antiphase when this input is not excited. Each output 96 is connected to a first input 98 of a corresponding analog adding circuit 100 known per se, from which the second inputs 102 are connected to one another and together with an output \ 0h of a generator I06 for generating an alternation-

The outputs I08 of the adder circuits 100 are each connected to an input 110 of a corresponding excitation amplifier 112, of which an output Wh is connected to a corresponding column conductor 35 in each case.

^{/ Λ>} In this part of the excitation circuit, the alternating voltage in i. The alternating voltage with the highest frequency is superimposed on the lowest frequency and the sum is fed to the two electrodes of the display device 80 after amplification.

If the information at the output 88-1 is a "0" during the first Pei'i odenabschnitts t. / \, I.e. the input 90-1 is not excited, is the high-frequency component of the alternating voltage on the fun. 1 scale 35 ~ 1 with the high frequency Tension on group leader 10-1 in antiphase, the is excited in this period segment with a pulse train. Corresponding to the description as given for FIG

received the playback element, which was assigned to the Gx ^ uppenleitc-r 10-1 and corresponds to column conductor 35-1, while t. / \ a

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AC voltage between the first and second electrodes with an amplitude V ₀ + "V ^ and is thereby brought into the OFF state. If the information offered is a" 1 ", it follows in a corresponding manner that the display element is in the ON state In a complete period t (n) 1 t / · \ + t „/ \ + ... + t. / \ the entire desired reproduction pattern is finally produced.

This exemplary embodiment of a display device according to the invention is by no means limiting. An expert can easily see that, for example, the alternating voltage of the lowest frequency can be superimposed on the alternating voltages at the group conductors 10 with a different ranking of the elements. Although the excitation circuit is generally designed with transistor circuits, this is by no means characteristic of the inventive idea. Most of the time, the adding circuits and the associated excitation amplifiers are combined into one circuit, as is, for example, the counter circuit h6 and the 1-OFF-m- Decoding circuit ^ 6 . Large parts of the excitation circuit can also be combined to form one or more integrated circuits.

Also the training of the multiplexer circuit and the Coupling of the same with the other elements of the excitation circuit is only given in summary form. Training

^ ⁵ as well as the coupling are influenced, among other things, by the form in which the information to be reproduced is available. Any adjustments that may be required are in the area of the expert.

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Claims (2)

9/22/1978 'X PHN 8929 PATENT PROVISIONS 1. display device, which contains a display screen with a number of display elements and an excitation circuit, which display screen with a liquid crystal, a first cover plate for the liquid crystal with a number of first electrodes on a side of the first cover plate facing the liquid crystal, a second cover plate with a number of second Electrodes are provided on a side of the second cover plate facing the liquid crystal and with 'optical aids for making a Riclitzus tandes of groups of molecules of the liquid crystal visible, the first and second cover plates being almost parallel to one another and with sealing means on the edges to enclose them of the liquid crystal are connected to one another between the cover plates and wherein a display element is formed by a part of the liquid crystal which lies between a part of a first electrode and an opposite part of a second E electrode is, and which excitation circuit with a first generator for generating a first alternating voltage with a high frequency, a second generator zürn. Generating a second alternating voltage with a low frequency, a selection circuit for alternating coupling of the first generator with each at least e Lnei ¹ first electrode , a control circuit for coupling the 909818/0897 22.9-78. 2 PHN 8929 first generator with the second electrodes and a coupling circuit for the permanent coupling of all display elements is provided with the second generator, wherein for a display element during the time in which the this display element corresponding first electrode is coupled to the first generator, the ¥ ecliselspaiinung to ι the second electrode corresponding to this display element with the alternating voltage at the first electrode for setting of a first level of liquid molecules crystal of this display element is in phase and in opposite phase for setting a second straightening state, characterized in that the cover plates with the Liquid crystal a display screen from the twisted nernatic Form type and the liquid crystal is of a type in which the liquid crystal has a dielectric relaxation having, the difference between the dielectric Constant in the middle direction of the long axes of the molecules and the dielectric constant in a transverse the direction lying thereon at the high frequency of the first alternating voltage has the sign of this difference opposite to the low frequency of the second alternating voltage Has a sign. 2. Display device according to claim 1, wherein the display elements through the switching circuit in time division multiplex ^{2δ are} excited and each of a number m first electrodes corresponds to one of the display elements in groups and the selection circuit couples the first electrodes alternately and in cyclic order with the first generator, characterized in that the amplitude of the alternating voltage with the high frequency at a first electrode .about a factor corresponding to the square root of m hüll or than the amplitude of the alternating voltage with the high frequency at a second electrode. 909818/0897